Inverters are frequently required in locations where AC Converter is unavailable from the mains. To convert DC power to AC power, an inverter circuit is utilised. There are two types of inverters. Inverters that are true/pure sine wave inverters, as well as inverters that are quasi or modified sine wave inverters. True/pure sine wave inverters are expensive, whereas modified or quasi inverters are less so.
These modified inverters generate a square wave, which is not suitable for powering sensitive electrical equipment. A basic voltage-driven inverter circuit is built here, which converts a 12V DC signal to single phase 220V AC utilising power transistors as switching devices.
- Principle Behind this Circuit
- Inverter circuit Using Transistors
- Circuit Diagram
- Components required
- Working
- Output Video
- 12v DC to 220v AC Converter Circuit Using Astable Multivibrator
- Circuit Design Explanation
- 12v DC to 220v AC Converter Circuit Operation
- Applications of 12v DC to 220v AC Converter Circuit
- Limitations
Principle Behind this Circuit
Every inverter circuit’s core concept is to generate oscillations from a given DC and then apply those oscillations across the primary of the transformer by amplifying the current. Depending on the number of turns in the primary and secondary coils, the primary voltage is then stepped up to a higher voltage.
Inverter circuit Using Transistors
A 12V DC to 220 V AC converter can also be designed using simple transistors. It can be used to power lamps up to 35W but can be made to drive more powerful loads by adding more MOSFETS.
The inverter implemented in this circuit is a square wave inverter and works with devices that do not require pure sine wave AC.
Circuit Diagram
Components required
- 12v Battery
- MOSFET IRF 630 -2
- 2N2222 Transistors
- 2.2uf capacitors-2
- Resistor
- 680 ohm-2
- 12k-2
- 12v-220v center tapped step up transformer.
- 2N2222 Datasheet
- IRF630 Datasheet
Working
The circuit is made up of three components: an oscillator, an amplifier, and a transformer. Because the frequency of the AC supply is 50Hz, a 50Hz oscillator is required.
This can be accomplished by building an Astable multivibrator that generates a 50Hz square wave. The oscillator is made up of R1, R2, R3, R4, C1, C2, T2, and T3.
Inverting square waves are generated by each transistor. The frequency will be determined by the values of R1, R2, and C1 (R4, R3, and C2 are all the same). The frequency of the square wave generated by the astable multivibrator can be calculated using the formula below.
F = 1/(1.38*R2*C1)
The inverting signals from the oscillator are amplified by the Power MOSFETS T1 and T4. These amplified signals are given to the step-up transformer with its center tap connected to 12V DC.
Output Video
https://www.youtube.com/embed/MhxtwV-0OwgThe turns ratio of the transformer must be 1:19 in order to convert 12V to 220V. The transformer combines both the inverting signals to generate a 220V alternating square wave output.
By using a 24V battery, loads up to 85W can be powered, but the design is inefficient. In order to increase the capacity of the inverter, the number of MOSFETS must be increased.
12v DC to 220v AC Converter Circuit Using Astable Multivibrator
Inverter circuits can use thyristors or transistors as switching devices. Power transistors are commonly utilised in low and medium power applications. The use of power transistors is due to their extremely low output impedance, which allows maximum current to flow at the output.
Switching is one of the most essential applications of a transistor. The transistor is biassed in the saturation and cut-off regions for this application.
Both the collector emitter and collector base junctions are forward biassed when the transistor is driven in the saturation region. The collector emitter voltage is lowest here, and the collector current is highest.
Another important aspect of this circuit is the oscillator. An important use of 555 Timer IC is in its use as an astable multivibrator.
An astable multivibrator produces an output signal which switches between the two states and hence can be used as an oscillator. The frequency of oscillation is determined by the values of capacitor and resistors.
Circuit Diagram
Circuit Components
- V1 = 12V
- R1 = 10K
- R2 = 150K
- R3 = 10Ohms
- R4 = 10Ohms
- Q1 = TIP41
- Q2 = TIP42
- D1 = D2 = 1N4007
- C3 = 2200uF
- T1 = 12V/220V step up transformer
Circuit Design Explanation
Oscillator Design:An astable multivibrator can be used as an oscillator. Here an astable multivibrator using 555 timer is designed. We know, frequency of oscillations for a 555 timer in astable mode is given by:
f = 1.44/(R1+2*R2)*C
where R1 is the resistance between discharge pin and Vcc, R2 is the resistance between discharge pin and threshold pin and C is capacitance between threshold pin and ground. Also the duty cycle of the output signal is given by:
D = (R1+R2)/(R1+2*R2)
Since our requirement is f =50Hz and D = 50% and assuming C to be 0.1uF, we can calculate the values of R1 and R2 to be 10K and 140K Ohms respectively. Here we prefer using a 150K potentiometer to fine tune the output signal.
Also a ceramic capacitor of 0.01uF is used between the control pin and ground.
Designing a Switching Circuit: Our major goal is to create a 220V AC signal. To allow the maximum amount of current to flow to the load, high-power transistors must be used. As a result, we employ a TIP41 power transistor with a maximum collector current of 6A and a base current equal to the collector current divided by the DC current gain. This results in a bias current of approximately 0.4A *10, or 4A. However, because this current exceeds the transistor’s maximum base current, we prefer a value less than the maximum base current. Assume that the bias current is 1A. The bias resistor is then calculated as follows:
Rb = (Vcc – VBE(ON))/Ibias
For each transistor, the VBE(ON) is about 2V. Thus Rb for each is calculated to be 10 Ohms. Since the diodes are used for biasing, the forward voltage drop across the diodes should be equal to the forward voltage drops across the transistors. For this reason, diodes 1N4007 are used.
The design considerations for both the PNP and NPN transistors are same. We are using a PNP power transistor TIP42.
Because the switching circuit’s output is a pulse width modulated output, it could contain harmonic frequencies other than the basic AC frequency. As a result, an electrolyte capacitor must be utilised to ensure that only the fundamental frequency passes through. We utilise a 2200uF electrolytic capacitor to filter out the harmonics in this case. It is preferable to utilise a step up transformer because 220V output is required. A 12V/220V step up transformer is utilised in this application.
12v DC to 220v AC Converter Circuit Operation
- The 555 timer connected in astable mode creates a square wave signal with a frequency of 50Hz when supplied by a 12V battery.
- When the output is set to logic high, diode D2 will conduct, and current will flow through diode D1, R3, and into the base of transistor Q1.
- As a result, transistor Q1 will be turned on. When the output is at a logic low level, diode D1 conducts and current flows via D1 and R4 to the base of Q2, turning it on.
- This allows the DC voltage to be created at random intervals across the main of the transformer. The capacitor maintains the signal’s frequency at the desired fundamental frequency.
- This 12V AC signal across the primary of the transformer is then stepped up to 220V AC signal across the transformer secondary.
Applications of 12v DC to 220v AC Converter Circuit
- This circuit can be used in cars and other vehicles to charge small batteries.
- This circuit can be used to drive low power AC motors
- It can be used in solar power system.
Limitations
- Since 555 Timer is used, the output may slightly vary around the required duty cycle of 50%, i.e. exact 50% duty cycle signal is hard to achieve.
- Use of transistors reduces the efficiency of the circuit.
- Use of switching transistors has the possibility of causing cross over distortion in the output signal. However this limitation has been reduced to some extent by the use of biasing diodes.
Note
Instead of 555 timer one can use any astable multivibrator. For example this circuits can also be build using 4047 astable multivibrator,whose output current is amplified and applied to the transformer.
